A = [0, 3, 5]
B = [5, 10, 15]
C = product(A, B)

for n in C:
   print(n)

(0, 5)
(0, 10)
(0, 15)
(3, 5)
(3, 10)
(3, 15)
(5, 5)
(5, 10)
(5, 15)

(0, 10)
(5, 5)

C = [p for p in product(A, B) if sum(p) == 10]

from collections import Counter
from itertools import chain
Counter(chain.from_iterable(C))

Counter({5: 2, 0: 1, 10: 1})

for n, freq in Counter({5: 2, 0: 1, 10: 1}).items():
    print('{}: {}'.format(n, freq))